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>>>finally tonight, something millions of americans happily turn to to relieve stress. it's in the news tonight and it's yoga. one of the country's best-known yoga teachers touched off a heated debate with a new book called bluntly, "how yoga can wreck your body." that's quite a charge. more from nbc's chris janzen.

>> reporter: all around the country, class is in session. far from its ancient origins in india, yoga has become a modern-day antidote to stress.

>>om.

>> reporter: its growing popularity is raising alarm.

>>as a spine specialist i see a ton of yoga injuries.

>> reporter: there are no definitive numbers, but a
columbia university
survey reveals injury to the lower back, shoulders, knees and neck and in some rare cases, yoga led to stroke.

>>certain
yoga positions
put a lot of stress on the
blood vessel
to the brain.

>> reporter: because of the risks, respected yoga instructor
ken black
suggest the vast majority of people should give up yoga all together.

>>e-mails are saying i wish i could have had the courage to tell my teacher i just got hurt.

>> reporter: he says many get hurt forcing themselves into positions they aren't ready to do. would you look at somebody like me and say, you're a classic, you should not be doing yoga?

>>what i would say is you need to do a lot of stuff to prepare for a yoga class.

>> reporter: black acknowledges that is not what yoga fans want to hear and they are fighting back.

>>if something bothers you, you back off or find a different way to ask a question.

>>yes there are dangers and risk. there are with everything. there are risks with love. that doesn't mean we don't go forward with something that can be so helpful.

>> reporter: the benefits from yoga can be significant, lowering stress and
blood pressure
, even improving your
sex life
. still, while experts disagree about the risks, they do advise caution, because as with love, sometimes yoga hurts.
chris jansing
, nbc news, new york.